NASA ST6 Build Strategy: Power-to-Weight, Aero Tradeoffs, and Rulebook Theory

Background Theory

While the Time Trial class shares a rulebook with Super Touring, racing for position at the pointy end in ST requires a different build philosophy than a pure TT/qualifying car.

Raw power usually wins.

Aero in ST6 is allowed, but is pretty heavily penalized. Mario Korf(Occam's Racer) has done lots of work, even in an actual wind tunnel, to determine/prove that even with such large penalties, one could go faster in pure laptime with no traffic, with aero.

I could do an airdam, wing(-1.0) and even a 4" splitter(-1.0). The tradeoff is not just the penalties applied to each of those mods, but also giving up the BTM credit(+.4).

For a TT6-only car, i'd probably(definitely?) build a cheaper/easier D or B series car with 20 less WHP, with aero. In ST5 and above, skipping the BTM credit and running an airdam and wing has proven to be the floor for competitive pace. Most records are being set with a splitter and the additional penalty.

For an ST6 car that needs to race w2w with potentially 175whp 2750lb cars, I am opting for a "missile car" philosophy.

I could have done drive by wire to make flat tuning much much easier, but opted against that extra cost and that P:W penalty. I do think that the penalty is too small, so it would have been and remains to be a future way to "buy speed" if I need/want to. This is the only performance I believe I am "leaving on the table" due to self imposed budget/scope limits.

Rulebook Highlights

This is a smattering of the specific lines relevant to my goals, definitely not an exhaustive list.

Class Target

Super Touring 6 (ST6) = “Adjusted Wt/HP Ratio” equal to, or greater than, 19.00:1

Tire Size Credit Rule

2) Vehicles with a Minimum Competition Weight (MCW) between 2749 lbs. and 2400 lbs. must have a measured tire NASA Section Width equal to or less than 257mm, fitting the 257mm tool. If the tire used fits the 226mm NASA Section Width tool, a positive (+) Modification Factor will be assessed. (note: Most currently produced size 225 tires should fit the 257mm section width tool on manufactured recommended wheel sizes)

Tire size credit:
NASA Section Width 226mm or less with MCW between 2400 lbs and 2749 lbs +0.3

Weight Factor Adjustment

• Less Than 2550 lbs = -0.3
• Less Than 2450 lbs = -0.4
• Less Than 2250 lbs = -0.5
• Less Than 2150 lbs = -0.6

Cable Throttle Credit

Manually, mechanically actuated throttle body (no electronic servo) = +0.3 (Does not apply to forced induction vehicles)

Wishbone / A-Arm Penalty

Suspension design utilizing upper “A-arm” or “wishbone” type control arms (front or rear) = -0.5

BTM (Base Trim Model) Aero Credit

BTM Aero (see 6.1.4) = +0.4

FWD Drivetrain Credit

FWD = +0.6

Ballast Limit

CCR 15.20—Up to three hundred (300) lbs. of added ballast is permitted.

Cage / Weight Rules

(I dont know if my cage is .095 or .120 wall, but im never getting close to 3000lbs anyways)

• 2501 - 3000 lbs → 1.750” x 0.095”
• 3001 - 4000 lbs → 1.750” x .120”

Build Strategy

By stacking Modification Factors, I can build this FWD, Base Trim Model, Cable Throttle, Small Tire Credit Civic into a 16.6lb per horsepower car, in a 19:1 class.

The important breakpoint is at 2400lbs, where the class allows 257mm tires. If I opt to run "undersized" tires that stay in the smaller 226mm template, I get a credit.

Very close to that 2400lb line is the next Weight Factor breakpoint, with another .1 P:W credit at 2450lbs.

Assuming I can acquire/build 300lbs of ballast, then I will be aiming for that 2450lb minimum.